1. Field of the Invention
The present invention relates to a control device for a servo die cushion which generates a force, on a slide of a press machine, using a servomotor as a drive source.
2. Description of the Related Art
It is known that a press machine, for bending, drawing or punching, etc., is provided with a die cushion mechanism, as an attached device, for applying a predetermined amount of force or pressure, to a movable support member (generally called a slide) supporting a first mold for press working, and force generated by another movable member supporting a second mold. The die cushion mechanism is generally configured such that the slide (or the first mold), moving in a mold-clamping direction, directly or indirectly collides with a movable element (generally known as a cushion pad) held at a predetermined pressure, and until the molding is finished, the cushion pad is moved with the slide while applying force or pressure to the slide. During this operation, it is possible to prevent wrinkles from forming in a workpiece to be pressed by, for example, clamping an area around a site of the workpiece to be pressed between the cushion pad and the slide.
Many conventional die cushion mechanisms use hydraulic or pneumatic units as driving sources. However, control by a hydraulic or a pneumatic unit can only be carried out under constant pressure. It is preferable that the pressure during drawing be varied in response to the amount of the drawing, however, the amount of pressure cannot be varied in the hydraulic or pneumatic unit.
In recent years, a die cushion mechanism using a servomotor as a driving source has been used to carry out force control, with a fast response, as described in Japanese Unexamined Patent Publication (Kokai) No. 10-202327. In the die cushion mechanism described in this publication, a cushion pad positioned below a slide of a press machine may be upwardly and downwardly moved by a servomotor, corresponding to the rise and fall of the slide. The servomotor operates by force control based on a force command value predetermined corresponding to the position of the cushion pad, and adjusts a force or pressure applied to the slide from the cushion pad, while moving the cushion pad with the slide. Collision and the pressure, between the slide and the cushion pad, are detected by detecting a load applied to an output axis of the servomotor via the cushion pad.
FIG. 12 is a graph showing ideal changes of the positions and the speeds of the slide and the die cushion (or the cushion pad), during a normal machining motion. First, the slide positioned at a top dead center thereof starts to go down (point “A”), collides with the die cushion positioned between the top dead center and a bottom dead center (point “B”), and reaches the bottom dead center with the die cushion (point “C”). Then the slide goes up and returns to the top dead center (point “D”). On the other hand, the die cushion starts to go up behind the slide (point “E”), and returns to an initial position thereof (point “F”). Such a series of operations are repeated. In FIG. 12, the speed of the slide and the die cushion when going up are represented as positive values.
In general, a speed command value for controlling the die cushion is calculated by one of the following equations:die cushion command speed=(force error)×(force gain)+(slide speed)  (1)die cushion command speed=(force error)×(first force gain)+Σ(force error)×(second force gain)+(slide speed)  (2)
where the force error is a difference calculated by subtracting an actual force (or a detected force value) from a force command value between the slide and the die cushion.
In either of the above equations, an absolute value of the die cushion speed command is smaller than an absolute value of the slide (detected) speed, when the force command is larger than the detected force (i.e., the force error>0). In a reverse case (the force error<0), the absolute value of the die cushion speed command is larger than the absolute value of the slide speed. At this point, just after the slide collides with the die cushion, a large amount of force is applied to the die cushion and the above detected force is substantially increased. Therefore, the difference between the detected force and the command force is increased (i.e., command force<<detected force). In particular, when the force gain is set to a relatively large value for a fast response, the difference between the absolute values of the die cushion speed command and the slide speed becomes larger, accordingly (i.e., |die cushion speed command|>>|slide speed|). In such a way, as shown in FIG. 13 which is an enlarged view of a portion “X” of FIG. 12, the die cushion speed converges with the slide speed, while alternately repeating overshoot and undershoot motions. In FIG. 13, the ideal profile of the die cushion speed is omitted, and instead, the die cushion speed command is shown.
As described above, the workpiece to be machined is pressed between the cushion pad and the slide. Since predetermined machining cannot be performed when the press force is reduced, it is important to prevent the press force from being reduced, in controlling the die cushion. However, the die cushion speed command represents the profile as shown in FIG. 13, the actual speed or detected speed of the die cushion represents a graph as shown in the upper part of FIG. 14. Accordingly, the detected force between the slide and the die cushion represents a graph as shown in the lower part of FIG. 14. In such a way, as shown in portion “Y” of FIG. 14, in a certain period of time, the detected force may be substantially smaller than the command force. In this period of time, the workpiece is not pressed between the slide and the cushion pad by sufficient force, whereby expected machining cannot be achieved.